It seems that Shakespeare did take many old themes and other people's plots and ideas to re-create in his own way. And what a way! Surely he who best uses an idea is most entitled to the credit.
There is nothing new under the sun, but there is always the new audience. For the majestic old poem of Spring, bound over in new covers of green, God creates fresh, eager young eyes and hearts each year. And not yet has he said to the year, "Do not attempt another spring—there have been so many before, you can but repeat their beauties." Then why should any mortal say to the poet or the author, "Do not try to write—it has all been said before."
Proceed, my young friend, and write what is in your heart. Nothing quite the same was ever in any heart before, and yet the greater part of it has been in all hearts, and will be in all hearts, so long as the world lasts.
Remember that when you write from the heart, it will go to the hearts of your readers: and when you write from your head it will go no lower than the head.
And if the critics score or ridicule you, consider yourself on the path to success.
If you have a message for the world, nothing and nobody can prevent you from delivering it.
He only fails who has nothing to say.